1. Field of the Invention
This invention relates to pre-load indicating fasteners in general and to a method and apparatus for tightening assemblies including such fasteners, in particular.
There are many situations when it is desired to tighten a particular joint assembly to a desired pre-load. In many situations such tightening is accomplished through the use of torque wrenches. However, due to the variation between torque and pre-load in the joint as a result of friction, this method is unsatisfactory in those areas where a high degree of accuracy is required. Pre-load indicating (hereinafter referred to as PLI) fasteners are commonly used when a high degree of accuracy is required with respect to tightening a joint assembly to a particular pre-load. PLI fasteners generally include a crushable element which is initially loaded when the fastener is tightened. There is a direct relationship between the desired pre-load and the yield point of the crushable element which can be calculated in advance. The yield point of the crushable element is predetermined in a number of ways including material selection and design configuration.
2. Description of the Prior Art
PLI fasteners are well known in the prior art. PLI fasteners include nut and bolt assemblies and washers with crushable or otherwise yielding elements.
U.S. Pat. No. 3,867,865 TWO-PART, TWO-MATERIAL FASTENING ELEMENT by Norman C. Dahl, issued Feb. 25, 1975 discloses a PLI bolt and PLI nut. Dahl teaches a crushable element in both the nut and the bolt, which is made of a material that is different from the primary load carrying structure of the nut or bolt.
U.S. Pat. No. 3,323,403 PRE-LOAD INDICATING WASHER by J. L. Waisman, issued June 6, 1967 teaches a PLI washer assembly. Waisman teaches one embodiment wherein a crushable washer is inserted between a bolt head and its adjacent structure. He also teaches another embodiment wherein two concentrically oriented washers are positioned between the bolt and its adjacent structure. The inner washer is the crushable element while the outer washer is a primary load carrying member. This second embodiment of Waisman has found heavy usage in practice, particularly for aerospace applications.
When initially tightening the dual washer assembly of Waisman the inner washer is loaded while the outer washer is not loaded and is rotatable due to a gap between the two washers. Initial tightening of the fastener causes the inner washer to be loaded in its elastic range. Subsequent tightening causes the inner washer to be loaded beyond its yield point causing it to plastically deform and be compressed outwardly and downwardly. During tightening the operator manually checks to see if the outer washer is free to rotate. Tightening is stopped when the outer washer is no longer free to rotate, indicating that sufficient plastic deformation of the inner washer has occurred.
Although PLI fasteners represent a considerable advance over tightening joint assemblies by monitoring the torque applied to the fastener during tightening, there have been problems associated with them. PLI fasteners are tightened using manual systems. The operator of such systems ceases to tighten when he feels the fastener enter the plastic range of the crushable element. This has resulted in two distinct problems.
When such PLI systems are manually tightened the operator stops tightening when he is sure that he is in the plastic range. This point is highly subjective and varies between individual operators as well as by an individual operator from joint to joint. The operator does not continue to tighten after he senses plastic deformation of the crushable element, since if he did so he would possibly enter the elastic range of the joint assembly. This would be undesirable as it would constitute an over-tightening of the joint assembly. Additionally, the operator would have no way of knowing to what extent he had tightened the joint assembly into its elastic range. Variation in bolt tension can be as much as .+-.7 percent of the bolt tension when the fastener assembly is tightened within the range of plastic deformation of the crushable element. In many instances this variation is greater than would otherwise be desired.
A second problem associated with prior art PLI fasteners is that of relaxation. As previously indicated, in tightening such fasteners the crushable element is only partially plastically deformed. This will, of course, result in a reduction of the static load being carried by the bolt. This represents a loosening or reduction in tightness of the assembled joint and is undesirable. In some applications it may result in the joint assembly having a significantly decreased propensity for enduring dynamic loading, thus increasing the potential of fatigue failure.
A third problem associated with PLI fasteners is that they have heretofore not lent themselves to automatic tightening systems. Most conventional tightening systems, as previously indicated, are based upon tightening the fastener until a preselected degree of torque resistance is met. These systems are, of course, not acceptable for tightening PLI fasteners since the primary purpose of the PLI fasteners is to eliminate the lack of precision provided by such torque tightening systems.
U.S. Pat. No. 3,982,419 APPARATUS FOR AND A METHOD OF DETERMINING ROTATIONAL AND LINEAR STIFFNESS, issued to John T. Boys on Sept. 28, 1976, teaches an automatic tightening system which is not based upon shut off at a torque value. The Boys system tightens until the gradient of the torque-rotation curve (i.e. the rate of change of torque with respect to angular rotation) reaches a predetermined relationship at the yield point of the joint assembly being tightened. The present invention includes a modification to this system which enables assemblies including PLI fasteners to be tightened until the crushable element of the PLI fastener is fully deformed and the joint assembly has been tightened slightly into its elastic region. The teachings in U.S. Pat. No. 3,982,419 are incorporated herein by reference.